Devices of this type, in which a set of conductors integral with the lead frame project from the resinous body on the side opposite the heat-dissipative metal strip to avoid possible short circuits, are available on the market under the designation TO-220. In such devices or packages, the projecting strip portion has a hole to be penetrated by a screw serving to fasten it to an underlying support serving, especially in the case of a power stage, as a heat sink. Theoretically, the rigid metal strip is flush with the underside of the resin body and is therefore in full-face contact with the heat sink. In practice, however, this strip is not perfectly straight but has undergone some deformation during prior handling so that the pressure exerted by the fastening screw upon the free end of the strip tends to elevate its embedded end above the supporting plate surface. This separation forces the major part of the thermal flux, originating from within the package, to pass along the strip to the vicinity of the fastening screw where the strip is in firm contact with the plate. The resulting increase in the thermal resistance of the flow path necessitates a considerable enlargement of the heat sink, in comparison with the theoretical requirement, for the purpose of effective dissipation of the thermal flux concentrated at the exposed strip portion.